Silver nanowires and preparation method thereof

ABSTRACT

Silver nanowire with a diameter smaller than 100 nm and an aspect ratio from 150 to 300 is disclosed. The preparation method of the silver nanowires is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 099146387, filed Dec. 28, 2010, the full disclosure of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The disclosure relates to a nanostructure and a preparation method thereof. More particularly, the disclosure relates to metal nanowires and a preparation thereof.

2. Description of Related Art

Silver nanowires have many unique properties. For example, silver nanowires can be applied in fabrics or medical products as an antimicrobial agent. Other applications include adding the silver nanowires into an adhesive resin to form conductive silver glue, which can provide the conductivity of conductive circuit. The silver nanowires also can be added into a polymer resin to form a conductive film, which can provide the conductivity of an electronic device by the network structures formed by the silver nanowires distributed therein. It is expected to use silver nanowires to prepare transparent conductive films for using on, such as electronic devices. The smaller the silver nanowires diameter is, the more transparent the conductive film is. Hence, it is quite important to mass produce silver nanowires with smaller diameter.

In the present, the silver nanowires can be mainly prepared by template method or chemical reduction method. In the chemical reduction method, a copper-mediated poylol process is usually used. However, only a small amount of silver nanowires can be obtained. For example, Younan Xia (J. Mater. Chem. 2008, 18, 473) disclosed the copper-mediated poylol process includes the following steps. Ethylene glycol was added in a reaction vessel to and the reaction system was heated to 150° C. Ethylene glycol solutions of cupric chloride, polyvinylpyrrolidone (PVP), and silver nitrate were sequentially added into the reaction vessel. The main product is silver naoparticles, and only a trace amount of silver nanowires is obtained. Moreover, the diameter of the silver nanowires mentioned above is all larger than 100 nm and could not provide the better optical transparency when utilized in a transparent conductive film. Therefore, Younan Xia's method cannot be applied to mass produce silver nanowires with diameter smaller than 100 nm, and hence has lower commercial value.

Accordingly, this invention provides a method to obtain silver nanowires with diameter smaller than 100 nm.

SUMMARY

In one aspect, the present invention is directed to a method of preparing silver nanowires. The method comprises the following steps. A copper salt and a silver salt are sequentially added to an ethylene glycol at a temperature of 140° C. to 160° C. to form a reaction solution. The reaction solution is maintained at a temperature of 140° C. to 160° C. for the subsequent two-step addition of PVP ethylene glycol solutions. A PVP ethylene glycol solution was prepared for two-step addition. A first addition step is performed to add 3 vol % to 4.5 vol % of the PVP ethylene glycol solution dropwisely to the reaction solution, and a second addition step is performed to add the rest of PVP ethylene glycol solution to the reaction solution at one time. Then, the reaction solution is cooled down and purified to obtain silver nanowires.

In another aspect, the present invention is directed to a silver nanowire having a diameter smaller than 100 nm and an aspect ratio of 150 to 300.

The above and other aspects of the invention will become better understood with regard to the following detailed description of the preferred but non-limiting embodiment(s). The following description is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flowchart of a method for preparing silver nanowires according to an embodiment of this invention.

DETAILED DESCRIPTION

Accordingly, this invention provides the silver nanowires with a diameter smaller than 100 nm and an aspect ratio from 150 to 300, and a preparation method thereof. In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing.

In J. Mater. Chem. 2008, 18, 473 mentioned above, the reaction temperature suddenly drops when ethylene glycol solution of silver nitrate is added. During the period of raising the reaction temperature back to 140° C., the reaction condition is disadvantageous to form the silver nanowires. Therefore, the main product is silver nanoparticels and only a trace of silver nanowires. Even though silver nanowires are obtained, the diameters of the silver nanowires are larger than 100 nm.

In this invention, the ethylene glycol solution of silver nitrate and cupric chloride is maintained at a temperature of 140° C. to 160° C. during the two-step addition of PVP ethylene glycol solution. In the first addition step, a small amount of PVP ethylene glycol solution is dropwisely added. In the second addition step, the rest of the PVP ethylene glycol solution is added at one time. Consequently, silver nanowires with diameter smaller than 100 nm and an aspect ratio of 150 to 300 are obtained.

FIG. 1 is a process flowchart of a method for preparing silver nanowires according to an embodiment of this invention. In step 110 of FIG. 1, a glycol is added into a reaction vessel to serve as a solvent and a reducing agent to reduce metal ions. The glycol can be selected from the ethylene glycol, propylene glycol, butylene glycol and a combination thereof, preferably ethylene glycol.

In step 120, the reaction system is preheated to above 140° C., such as 140° C. to 160° C. Then, the reaction system needs to be maintained at a temperature of 140° C. to 160° C., such as 148° C. to 155° C.

In step 130, a copper salt is added into the reaction vessel to form crystal seeds. The copper salt can be selected from the group of cupric chloride, cupric bromide, and a combination thereof, preferably cupric chloride. The copper salt can be added into the reaction vessel in a form of a glycol solution of the copper salt. The concentration of the copper salt in the glycol solution can be 0.003 M to 0.016 M, for example, such as 0.004 M.

In step 140, a silver salt is added into the reaction vessel to start forming silver nanostructures. The silver salt can be, for example silver nitrate. A glycol solution of the silver salt can be prepared, and then added into the reaction vessel. The concentration of the silver salt in the glycol solution can be 0.1 M to 0.4 M, for example, such as 0.12 M.

In step 150, a small amount of polyvinylpyrrolidone (PVP) are dropwisely added into the reaction vessel. The PVP can be added into the reaction vessel as glycol solution of PVP. The concentration of PVP glycol solution can be 0.1 M to 0.6 M, such as 0.2 M to 0.4 M. In this step, the addition amount of the PVP glycol solution can be 3 vol % to 4.5 vol %, such as 3.5 vol % to 4 vol % of the total amount of PVP glycol solution. The addition time at constant dropping rate can be 2 minutes to 4 minutes.

The molar ratio of the PVP to the copper salt can be 750 to 1500, such as 800 to 1200.

The molar ratio of the PVP to the silver salt can be greater than 2, such as 2 to 5. If the addition amount of the silver salt is too much, the diameter of the silver nanowires will be larger, and the residual unreacted silver salt will be obtained. A slight excess of silver salt does not affect the aspect ratio of the silver nanowires much, the molar ratio of the PVP to the silver salt is only needed to be greater than 2.

In this step, some silver nanowires have been started to form in the reaction vessel. The PVP is adsorbed on a certain surface of the silver nanowires via the oxygen and nitrogen atoms of the vinylpyrrolidone monomer to be a shape control agent of silver nanowires.

In step 160, the rest of the PVP glycol solution is added into the reaction vessel at one time to let the silver nanowires grow faster along the longitudinal direction than along the lateral direction, while the temperature of the reaction system is still maintained at the constant temperature of 140° C. to 160° C. Hence, silver nanowires with a diameter smaller than 100 nm and an aspect ratio of 150 to 300 can be obtained.

In step 170, reacting for a period of time, silver nanowires with high aspect ratio can be obtained. Both longer and shorter reaction, time can decrease the aspect ratio of silver nanowires. When the reaction time is too short, the length of the silver nanowires is short due to insufficient reaction time. When the reaction time is too long, since the reaction system is exposed to air, the length of the silver nanowires is decreased due to oxidative etching. Moreover, the diameter of the silver nanowires is increased.

In step 180, the reaction system is stopped heating after the reaction finished. Next, the silver nanowires are purified. The purification method can be, for example, centrifugation.

The obtained silver nanowires above can be added into some polymer resin to form conductive films or conductive silver glue. The silver nanowires can also be applied to fabrics or medical products as an antimicrobial agent.

Some embodiments are provided below to further illustrate this invention, but this invention is not limited thereto.

Example 1

The preparation steps of Example 1 are described as follow. First, 50 ml of ethylene glycol (purity 95%, commercial code TG-073-000000, from Echo Chemical Co., Ltd., Taiwan) in a reaction vessel was preheated. Then, 1 ml of 0.004 M ethylene glycol solution of cupric chloride (purity 97%, commercial code 0341-3250, from SHOWA, Japan) and 20 ml of 0.12 M ethylene glycol solution of silver nitrate (purity 99.8%, commercial code Sl-0180-34-1052, from Fullin Chemical Co., Ltd., Taiwan) were sequentially added into the reaction vessel, and the temperature of the reaction vessel was maintained at 150° C.

Next, PVP (MW 55,000, commercial code 85656-8, from Aldrich Co., Ltd., US) was dissolved in 20 ml ethylene glycol to prepare 0.36 M ethylene glycol solution of PVP for two-step addition. At the first addition step, 3.75 vol % of the PVP ethylene glycol solution was dropwisely added in to the reaction vessel for 3 minutes. At the second addition step, the rest of the PVP ethylene glycol solution was added at one time. Then, the reaction system was allowed to react for 30 minutes.

Finally, the reaction system was stopped heating to cool down the reaction system. Centrifugation was used to purify the silver nanowires to obtain silver nanowires with a length of 9 μm to 19 μm, a diameter of 55 nm to 90 nm and an aspect ratio of 164 to 211.

Example 2

The preparation steps of Example 2 are the same as Example 1, except that the amount of PVP ethylene glycol solution added at the first addition step is 6.25 vol % for 5 minutes. Finally, silver nanowires with length of 7 μm to 10 μm, diameter of 55 nm to 80 nm, and an aspect ratio of 125 to 127 were obtained.

The results of Examples 1-2 are listed in Table 1 below. From Table 1, the aspect ratio of silver nanowires was decreased by increasing the addition amount of PVP ethylene glycol solution at the first addition step.

TABLE 1 Dropwise addition amount of PVP and the Characteristics of silver nanowires Dropwise Characteristics of Dropwise addition silver nanowires PVP/Ag/Cu addition time amount of Length Diameter Aspect yield Examples Molar ratio of PVP PVP (vol %) (μm) (nm) ratio (%) 1 1800/600/1 3 3.75 9-19 55-90 164-211 90 2 1800/600/1 5 6.25 7-10 55-80 125-127 90

Example 3

The preparation steps of Example 3 are described as follow. First, 50 ml of ethylene glycol in a reaction vessel was preheated. Then, 2 ml of 0.004 M ethylene glycol solution of cupric chloride and 20 ml of 0.12 M ethylene glycol solution of silver nitrate were sequentially added into the reaction vessel, and the temperature of the reaction vessel was maintained at 150° C.

Next, PVP was dissolved in 20 ml ethylene glycol to prepare 0.36 M ethylene glycol solution of PVP. At the first addition step, 3.75 vol % of the PVP ethylene glycol solution was dropwisely added in to the reaction vessel for 3 minutes. At the second addition step, the rest of the PVP ethylene glycol solution was added at one time. Then, the reaction system was allowed to react for 8 minutes.

Finally, the reaction system was stopped heating to cool down the reaction system. Centrifugation was used to purify the silver nanowires to obtain silver nanowires with a length of 8 μm to 13 μm, a diameter of 48 nm to 70 nm and an aspect ratio of 167 to 186.

Example 4

The preparation steps of Example 4 are the same as Example 3, except that the reaction time was 15 minutes. Finally, silver nanowires with length of 7 μm to 19 μm, diameter of 45 nm to 65 nm, and an aspect ratio of 156 to 292 were obtained.

Example 5

The preparation steps of Example 5 are the same as Example 3, except that the reaction time was 30 minutes. Finally, silver nanowires with length of 6 μm to 16 μm, diameter of 45 nm to 57 nm, and an aspect ratio of 133 to 281 were obtained.

Example 6

The preparation steps of Example 6 are the same as Example 3, except that the reaction time was 60 minutes. Finally, silver nanowires with length of 10 μm to 15 μm, diameter of 45 nm to 75 nm, and an aspect ratio of 200 to 222 were obtained.

Example 7

The preparation steps of Example 7 are the same as Example 3, except that the reaction time was 120 minutes. Finally, silver nanowires with length of 8 μm to 13 μm, diameter of 50 nm to 75 nm, and aspect ratio of 160 to 173 were obtained.

The results of Examples 3 to 7 are listed in Table 2 below. From Table 2, silver nanowires with greater aspect ratio could be obtained by adjusting the reaction time. If the reaction time is too long or to short, the aspect ratio of the silver nanowires would be decreased.

TABLE 2 Reaction time and the characteristics of silver nanowires Characteristics of Reaction silver nanowires PVP/Ag/Cu time Length Diameter Aspect yield Examples Molar ratio (minutes) (μm) (nm) ratio (%) 3 900/300/1 8 8-13 48-70 167-186 90 4 900/300/1 15 7-19 45-65 156-292 90 5 900/300/1 30 6-16 45-57 133-281 85 6 900/300/1 60 10-15  45-75 200-222 90 7 900/300/1 120 8-13 50-75 160-173 85

Example 8

The preparation steps of Example 8 are the same as Example 1, except that the addition amount of the 0.004 M ethylene glycol solution of cupric chloride was 4 ml. Finally, silver nanowires with length of 9 μm to 15 μm, diameter of 45 nm to 70 nm, and an aspect ratio of 200 to 214 were obtained.

Example 9

The preparation steps of Example 9 are the same as Example 1, except that the addition amount of 0.12 M ethylene glycol solution of silver nitrate was 30 ml. Finally, silver nanowires with length of 11 μm to 21 μm, diameter of 60 nm to 75 nm, and an aspect ratio of 183 to 280 were obtained.

The results of Examples 1, 5, 8, and 9 are listed in Table 3 below. In Table 3, the reaction time of all examples was 30 minutes, and the molar ratio of silver to copper was increased from 150 to 600. From the data of the characteristics of the silver nanowires, it can be known that the excess amount of silver salt does not affect the characteristics of the silver nanowires, only unreacted silver salt was left in the reaction system.

TABLE 3 Silver addition amount and the characteristics of the silver nanowires. Characteristics of Reaction silver nanowires PVP/Ag/Cu time Length Diameter Aspect yield Examples Molar ratio (minutes) (μm) (nm) ratio (%) 1 1800/600/1  30 9-19 55-90 164-221 90 5 900/300/1 30 6-16 45-57 133-281 85 8 450/150/1 30 9-15 45-70 164-214 90 9 900/450/1 30 11-21  60-75 183-280 80

From the examples above, a two-step PVP ethylene glycol solution addition was provided to maintain the reaction system at a constant temperature to produce silver nanowires by the polyol process. That is, the dropwise addition amount of the PVP ethylene glycol solution and the reaction time were well controlled to obtain silver nanowires with diameter smaller than 100 nm and aspect ratio of 150 to 300, and the yield of the silver nanowires was above 80%.

While the invention has been described by way of example(s) and in terms of the preferred embodiment(s), it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 

1. A method of preparing silver nanowires, the method comprising: sequentially adding a copper salt and a silver salt to a glycol at a temperature of 140° C. to 160° C. to form a reaction solution; preparing PVP (polyvinylpyrrolidone) glycol solution; performing a first addition step to add 3 vol % to 4.5 vol % of the PVP glycol solution dropwisely to the reaction solution while the reaction solution is maintained at a temperature of 140° C. to 160° C.; performing a second addition step to add the rest of the PVP glycol solution to the reaction solution at one time while the reaction solution is maintained at a temperature of 140° C. to 160° C.; cooling the reaction solution; and purifying silver nanowires.
 2. The method of claim 1, wherein the copper salt is selected from the group of CuCl₂, CuBr₂, and a combination thereof.
 3. The method of claim 1, wherein the silver salt is AgNO₃.
 4. The method of claim 1, wherein the first PVP addition step is performed at a constant dropping rate in 2 to 4 minutes.
 5. The method of claim 1, wherein the molar ratio of the PVP to the copper salt is 750 to 1500, and the molar ratio of the PVP to the silver salt is 2 to
 5. 6. The method of claim 1, wherein the temperature of the reaction solution is 148° C. to 155° C.
 7. The method of claim 1, wherein the purifying method is centrifugation.
 8. The method of claim 1, wherein the glycol can selected from the group of ethylene glycol, propylene ethylene glycol, butylene ethylene glycol and a combination thereof.
 9. A silver nanowire having a diameter smaller than 100 nm and an aspect ratio of 150 to
 300. 